1. Field of the Invention
This invention relates to improvements in centrifugal pump performance and reliability. More particularly, but not by way of limitation, the invention pertains to apparatus for minimizing flow recirculation effects induced by wear ring leakage flow.
2. Description of the Prior Art
A wide variety of fluids (including water, hydrocarbons, slurries, air, natural gas, and other liquid and gaseous fluids) are pumped using centrifugal pumps. Centrifugal pumps generally provide steady flow at uniform pressures without the pressure surges characteristically found with reciprocating pumps. Consequently, they are applied in a diversity of processes requiring uniform pressures. Examples of typical processes requiring centrifugal pumps include steam power plants, water supply plants, oil refineries, chemical plants, steel mills, food processing factories, mining operations, dredging operations, and hydraulic power systems.
A typical prior art centrifugal pump is illustrated in FIGS. 1 and 2. The pump 10 has an impeller 22 having a plurality of impeller vanes 24. The impeller 22 is mounted on a shaft 28. The impeller 22 and the shaft 28 are rotatably mounted in a housing or casing 12. A motor or other power source (not shown) is used to rotate the shaft 28.
Fluid is pumped through a centrifugal pump by rotating the impeller 22 and the shaft 28. This rotation creates a suction at the inlet 13 of the pump (also known as the "eye side" of the pump), causing the fluid to travel into the pump. The impeller 22 then forces the fluid radially outwardly through the impeller 22, past the discharge tips 25 of the impeller vanes 24, and into the discharge annulus 17 leading to the discharge port 16. As illustrated in FIGS. 1 and 2, discharge annulus 17 has a volute shape; however, other centrifugal pumps utilize a discharge annulus having a uniform cross section. The discharge port 16 is connected to, or in fluid communication with, an output pipe or conduit (not shown) through which the fluid is pumped. The inlet 13 of the pump is typically connected to a pipe or conduit (not shown) through which fluid flows toward the centrifugal pump.
The fluid passes through the pump predominantly in a flow pattern hereinafter identified as the "primary fluid flow." As illustrated in FIG. 2, the primary fluid flow P through the pump 10 passes from the inlet 13 towards the impeller 22 while remaining substantially parallel to the longitudinal centerline of pump shaft 28. The primary fluid flow subsequently undergoes an approximately 90 degree change of direction in passing through the impeller 22 with the fluid propelled radially outwardly towards the discharge annulus 17. The fluid then flows through the discharge annulus to the discharge port and out of the pump.
A typical prior art centrifugal pump utilizes at least one controlled leakage joint between the rotating impeller and the stationary housing to reduce wear due to friction. Typically, most pumps utilize both an eye side controlled leakage joint 30 and a hub side controlled leakage joint 32. A controlled leakage joint permits a small amount of the fluid to pass or "leak" between its moving and stationary surfaces so as to reduce friction.
Leakage flow through the hub side controlled leakage joint 32 results in a portion of the fluid getting into the cavity 33 (see FIG. 3) between the hub side of impeller 22 and housing 12. This fluid must be permitted to flow back to the eye side of the impeller 22 in order to hydraulically balance the impeller 22. Thus, impellers typically include one or more balance holes 34 which permit this flow. At normal pump operating speeds, the flow through the balance holes does not create any significant problems. However, as illustrated in FIG. 3, at low pump flow rates the balance hole leakage flow causes turbulence, commonly referred to as leakage flow recirculation, in the pump inlet 13 and impeller eye 18 which substantially reduces the overall efficiency of the pump.
Leakage flow also occurs through the eye side controlled leakage joint 30. This leakage flow results from the pressure differential between the fluid in the discharge annulus 17 and the fluid in the pump inlet 13. The higher pressure at the discharge annulus 17 relative to the pressure at the pump inlet 13 generates a leakage flow towards the pump inlet 13. The eye side controlled leakage joint 30 serves to control this eye side leakage flow. However, as illustrated in FIG. 3, at low pump flow rates the eye side controlled leakage flow also causes turbulence (leakage flow recirculation) in the pump inlet 13 and impeller eye 18 which additionally contributes to reducing pump efficiency. Additional information on the effects of leakage flow on pump efficiency may be found in Centrifugal and Axial Flow pumps, Stepanoff, A. J., 2nd edition, John Wiley & Sons, Ch. 10.
At sufficiently low pump flow rates, significant adverse consequences may result from leakage flow recirculation. Problems arising from leakage flow recirculation include broken shafts, short seal life, failed bearings, high vibration, noisy operation, flow instability (i.e., surging), and cavitation damage on the pressure side of the impeller vanes.
In addition to leakage flow recirculation, two other types of recirculation, suction recirculation and discharge recirculation, can adversely affect a centrifugal pump's performance. These types of recirculation may induce problems similar to those caused by leakage flow recirculation. Suction recirculation is a flow reversal where fluid flows in the center of the inlet 13 toward the pump, while fluid along the periphery of inlet 13 reverses and flows away from the pump. Discharge recirculation is reversal of flow at the impeller vane discharge tips 25. An extended discussion of suction and discharge recirculation is presented in "Recirculation in Centrifugal Pumps," Fraser, W. H., Winter Annual Meeting of ASME, Nov. 15-20, 1981.
The prior art has focused on arresting suction and discharge recirculation. For example, Cliborn's U.S. Pat. No. 2,865,297 illustrates a device which reinjects fluid from the pump discharge to the impeller inlet for preventing suction and discharge recirculation. McCoy's U.S. Pat. No. 4,492,516 improves on Cliborn's teachings with an apparatus and method used to control the angle and direction at which the fluid is reinjected. Both Cliborn and McCoy limit the scope of their respective inventions to resolving the suction and discharge recirculation problems.
Another mechanism for arresting suction recirculation is an inducer (i.e. a screw like device attached to the front of the impeller) which serves to enhance fluid flow into the impeller. Jackson's U.S. Pat. No. 3,504,986 and Berman's U.S. Pat. No. 3,723,019 disclose the use of an inducer to combat suction recirculation. An inducer, however, counteracts recirculation effects under a principle similar to that applied in Cliborn's and McCoy's discharge fluid reinjection devices, namely, by increased pump flow rate to the impeller.
Another method commonly used to minimize recirculation effects is to operate the pump at an artificially high pump flow rate. Particularly where low process stream flow rates are required, the high pump flow rate needed to minimize recirculation effects is achieved by a recycle line (not shown) communicating between the discharge port 16 and pump inlet 13. The recycle, thereby, permits release of only that portion of the pumped fluid required for maintaining the desired process flow rate, despite the high pump flow rate.
In all the aforementioned cases, the recirculation effect is minimized by increasing the pump flow rate through the impeller with reinjection or inducer means or recycling flow from discharge to inlet. However, these means are inefficient by virtue of the additional energy and equipment required. Accordingly, a need exists for a means of correcting leakage flow recirculation effects at low pump flow rates without the necessity for using additional energy and equipment. The present invention provides apparatus for improving a centrifugal pump's inherent resistance to producing leakage flow recirculation.